Support a texture mask for off-road #5861

The way the current off-road system works depends purely on the texture the kart is over. This has the advantage of relative simplicity, but it creates issues in some situations.

That's especially the case for texture transition, for example between a main road and grass or sand on the side.

It also makes it impossible to have small alternating patches of on-road and off-road terrain without directly specifying a different texture for each surface.

The idea of a texture mask would be to check over which part of the texture the kart is driving, and see if it counts as off-road or not according to the mask, allowing more complicated off-road terrain shapes with a smaller workload.

While the high-level concept appears logical, I haven't investigated how easy it is to determine which part of the texture a raycast hits, so that's the key point to investigate before the rest of the code is created.

The off-road mask would only work on version 8 tracks (Evolution's format).